1. Field of the Invention
This invention relates to a pulling apparatus and a pulling method for single crystal production. More particularly, it relates to a pulling apparatus and a pulling method based on an automatic melt level control system in which a detection spot formed by a laser beam is imaged to produce image signals with which to control the melt level.
2. Description of the Related Art
A silicon single crystal called an ingot used to make silicon wafers is generally produced from polycrystalline silicon by Czochralski method (CZ method).
In the CZ method, the silicon melt level in a quartz glass-made crucible falls according as a silicon single crystal is pulled up. For obtaining high quality single crystals, it is important to control the position of the melt level constant in relation to a heater by driving the crucible shaft to lift the crucible. It is necessary to detect the melt level before the melt level can be controlled. Various methods have been proposed for melt level detection.
JP-A-6-116083 discloses a melt level control system in which a mechanical reference mark is provided on the inner material of a heating furnace, and a reflected image of the reference mark formed on the surface of a melt is sensed with a CCD camera.
In this system, a carbon heater disposed around the periphery of a quartz glass crucible is the light source for reflecting the reference mark on the melt surface. The brightness of the heater depends on the power consumed by the heater. However, the power is not constant because it is controlled based on the temperature of the silicon melt. Therefore, under some situations, for example, when the power reduces, the image formed on the melt surface becomes unclear and cannot be binalized, resulting in a failure to detect the melt level.
JP-A-64-83595 discloses a diameter measuring system which does not use a carbon heater as a light source, in which the distance between a detector and a melt surface is measured by means of a laser beam and an image sensor, and the crucible shaft is lifted to adjust that distance at a set value.
A melt level control system which does not rely on a carbon heater as a light source is also disclosed in JP-A-5-194079, in which the diameter of a spot formed by a laser beam is detected with a CCD camera, and a servo motor for a crucible shaft lift mechanism is operated so as to set the deviation of the detected diameter from a set diameter within a critical value.
Under strict safety standards, it is not easy to handle a high-output laser in these melt level control systems. It is desirable to limit the output of a laser unit used in the control system at 5 mW or less.
In order to study laser light that will form a spot directly on a silicon melt surface similarly to the conventional techniques, the inventors of the present invention examined light generated within a furnace. As a result, it was confirmed that the light emitted within a ceramic furnace while a crystal is being pulled up has a high intensity in a wavelength range of from about 500 to 700 nm as shown in FIG. 11. They tested at first a red laser having an oscillation wavelength of 640 to 770 nm which is relatively easy to handle and inexpensive but failed to form an adequate spot with an output of 5 mW. They also tried a green laser beam having a wavelength of 490 to 550 nm but found it incapable of forming a spot directly on the melt surface.
As a result of further investigations, the inventors have found that a spot can be formed on a melt surface by directing a laser beam whose wavelength is 550 nm or less, i.e., does not overlap with the wavelengths of the light emitted in the furnace, to a specific position of the melt surface.
That is, they have found that a spot can be formed on a melt surface even with a weak output by irradiating the melt surface with a laser beam having a wavelength of 550 nm or less such as a green laser beam.
In particular, they have found that a spot can be formed on the surface of a melt by emitting a laser beam having a wavelength of 550 nm or less from above a specific cylindrical cover (preferably made of an opaque ceramic material) placed above the melt. The cylindrical cover is composed of a cylindrical body portion (inclusive of an inverted conical body) having an upper and a lower opening for allowing a growing single crystal to pass through and a horizontal portion which extends from the lower end of the cylindrical body portion in the radial direction either inwardly or outwardly and faces the melt surface with a gap therebetween. The laser beam illuminates the melt surface below the horizontal portion of the cylindrical cover to form a spot on the melt surface near the illuminated position.
An object of the present invention is to provide a pulling apparatus and a pulling method in which a melt level can be detected accurately to enable automatic melt level control for obtaining a high-quality single crystal and which are safe and easy to handle or carry out.
The present invention provides, in its first aspect, a single crystal pulling apparatus comprising a quartz glass crucible provided in a chamber vessel and a heater for heating polycrystalline silicon put in the crucible to produce a melt, in which a seed crystal is dipped and lifted to pull a single crystal, the apparatus having a laser unit which emits a beam having a wavelength of 550 nm or less to form a spot on the surface of the melt, an image capturing unit for capturing the spot to produce image signals, a control unit which receives the image signals as binalized and processed by an image processing means, and a crucible shaft lift which lifts a crucible shaft under instructions from the control unit to adjust the melt level.
The present invention provides, in its second aspect, a single crystal pulling apparatus comprising a quartz glass crucible provided in a chamber vessel and a heater for heating polycrystalline silicon put in the crucible to produce a melt, in which a seed crystal is dipped and lifted to pull a single crystal, the apparatus having a cylindrical cover placed above the crucible to surround a growing single crystal, a laser unit which emits a beam having a wavelength of 550 nm or less to form a spot on the surface of the melt, an image capturing unit for forming an image of the spot to produce image signals, a control unit which receives the image signals as binalized and processed by an image processing means, and a crucible shaft lift which lifts a crucible shaft under instructions from the control unit to adjust the melt level, the cylindrical cover being composed of a cylindrical body portion having an upper and a lower opening for allowing a growing single crystal to pass through and a horizontal portion which extends from the lower end of the cylindrical body portion in the radial direction either inwardly or outwardly and faces the melt with a gap therebetween.
In a preferred embodiment of the first and the second aspects of the present invention, the laser beam illuminates a light transmitting object provided between the laser unit and the melt level to form a virtual image of a spot on the melt surface.
In a still preferred embodiment, the light transmitting object is provided in the horizontal portion of the cylindrical cover.
In another preferred embodiment, the laser beam is a green laser beam or a blue laser beam.
In yet another preferred embodiment, the automatic melt level control system comprising the image capturing unit, the image processing means, and the control unit also serves for automatic diameter control.
The present invention also provides, in its third aspect, a single crystal pulling method comprising charging a semiconductor raw material into a quartz glass crucible that can be lifted in a chamber vessel, heating the raw material into a melt, dipping a seed crystal in the melt, and lifting the seed crystal to pull a single crystal, wherein a spot for melt level detection is formed on the melt surface by a laser beam having a wavelength of 550 nm or less, the spot is imaged to produce image signals, which are binalized and processed into melt level data, and the crucible is lifted based on the melt level data to control the melt level.
In a preferred embodiment of the third aspect, the laser beam is directed to a light transmitting object provided in the path of the laser beam to form a virtual image of a spot on the melt surface.
The present invention also provides, in its fourth aspect, a single crystal pulling method comprising charging polycrystalline silicon into a quartz glass crucible that can be lifted in a chamber vessel, heating the polycrystalline silicon into a melt, pulling down a seed crystal through a hole of a cylindrical cover disposed above the crucible, dipping the seed crystal in the melt, and lifting the seed crystal to pull a single crystal, wherein a laser beam having a wavelength of 550 nm or less is projected to the melt surface and reflected on the melt surface to form a spot on the back side of a horizontal portion extending from the lower end of the cylindrical cover, the spot is imaged to produce image signals, which are binalized and processed into melt level data, and the crucible is lifted based on the melt level data to control the melt level.